This invention relates to an improvement of an optical scanning device in which a hologram record medium is rotated so as to move an optical spot on the scanning surface through diffraction.
In general, an optical scanning device of this type employs a hologram record medium, or a rotary body, on the same circumference of which a plurality of interference fringes formed by interference between spherical waves or by interference between a spherical wave and a plane wave are recorded. While a laser beam is applied to the record medium in a predetermined direction, the record medium is rotated, whereby an optical spot on the scanning surface is displaced through hologram diffraction.
With the optical scanning device, in order to provide the optical spot at a predetermined position in the auxiliary scanning direction, on the scanning surface, it is necessary that the incident angle of the laser beam with respect to the hologram surfaces of the hologram record medium is maintained unchanged at all times. In this case, the light source causes no problem because it is fixed. However, the hologram record medium is liable to swing with respect to its rotary axis because its construction is of a rotary mechanism. Therefore, the hologram record medium may cause a problem in that the incident angle of the irradiating beam with respect to the hologram surface may become deviated and shift the position of the optical spot in the auxiliary scanning direction.
The following technical concept is known in the art as disclosed by Japanese Patent Laid-Open No. 15155/1978. In the prior art, when the hologram surface is inclined around the vicinity of the incident position of a beam applied thereto, the condition that the angle of the diffraction beam is maintained unchanged is obtained by analysis. According to the condition thus obtained, the angle of the irradiating beam with respect to the hologram surface is made equal to the angle of the diffraction beam, so that even if the hologram surface is inclined, the angle of the diffraction beam reproduced is maintained unchanged.
However, where the prior art is applied to the abovedescribed optical scanning device, the following difficulties occur: In this case, the inclination of the hologram surface is not relative the hologram surface itself but to the rotary shaft of the hologram record medium. Accordingly, the above-described condition cannot be satisfied, and the shift of the scanning beam position due to the inclination of the hologram record medium cannot be prevented at all.
This will be described with reference to FIG. 1. It is assumed that a conical hologram record medium 1 is inclined by .DELTA..theta. with respect to the rotary shaft 2 thereof. In this case, the hologram surface is also inclined by .DELTA..theta., and at the same time a position O where the principal ray of an object light is recorded is shifted to a point O'. If, in this case, the hologram is so set that, as in the prior art, the incident angle of the irradiating beam is equal to the angle of the diffraction beam (the incident angle of an object light and the incident angle of a reference light in recording a hologram being equal to each other and being on both sides of a normal to the exposure surface), then even if the hologram surface is inclined by .DELTA..theta., the angle of the principal ray diffracted by the inclination is maintained unchanged, and therefore lines PO and P'O are in parallel with each other. As a result, the scanning optical spot is shifted from the position P to the position P'.
As is clear from the above description, if the hologram record medium is inclined with respect to its rotary shaft by mechanical vibration or the like, then the optical spot is displaced in the auxiliary scanning direction, with the result that the scanning line pitch is irregular.